Method and device for producing formates and the use thereof

ABSTRACT

A process for preparing acid formates which comprises
         (a) partially hydrolyzing methyl formates with water;   (b) separating off by distillation methyl formate and methanol from the reaction mixture obtained in process stage (a), forming a stream comprising formic acid and water; and   (c) combining the stream comprising formic acid and water from the process stage (b) and the corresponding formate, forming a mixture comprising the acid formate and water,
 
an apparatus for their preparation and their use.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a National Stage application ofPCT/EP2003/008400, filed Jul. 30, 2003, which claims priority fromGerman Patent Application No. DE 102 37 380.9, filed Aug. 12, 2002.

The present invention relates to a process and an apparatus forpreparing acid formates starting from methyl formate, water and a basiccompound.

In addition, the invention relates to the use of the acid formates forpreserving and/or acidifying plant and/or animal materials, for treatingbiowastes and as an additive in animal nutrition and/or as growthpromoters for animals.

Acid formates have an antimicrobial activity and are used, for example,for preserving and for acidifying plant and animal materials, forinstance grasses, agricultural products or meat, for treating biowastesor as an additive for animal nutrition.

Acid formates and preparation methods for these have long been known.Thus, Gmelins Handbuch der anorganischen Chemie [Gmelin's Handbook ofInorganic Chemistry], 8^(th) edition, Number 21, pages 816 to 819,Verlag Chemie GmbH, Berlin 1928 and Number 22, pages 919 to 921, VerlagChemie GmbH, Berlin 1937 describes the synthesis of sodium diformate andof potassium diformate by dissolving sodium formate and potassiumformate in formic acid. The crystalline diformates may be obtained bydecreasing the temperature and by evaporating off excess formic acid.

DE 424 017 teaches preparing acid sodium formates having varying acidcontent by introducing sodium formate into aqueous formic acid in anappropriate molar ratio. By cooling the solution the correspondingcrystals can be obtained.

According to J. Kendall et al., Journal of the American ChemicalSociety, Vol. 43, 1921, pages 1470 to 1481, acid potassium formates maybe obtained by dissolving potassium carbonate in 90% strength formicacid, forming carbon dioxide. The corresponding solids can be obtainedby crystallization.

U.S. Pat. No. 4,261,755 describes preparing acid formates by reacting anexcess of formic acid with the hydroxide, carbonate or bicarbonate ofthe corresponding cation.

WO 96/35657 teaches preparing products which contain disalts of formicacid by mixing potassium formate, hydroxide, carbonate or bicarbonate,sodium formate, hydroxide, carbonate or bicarbonate, cesium formate,hydroxide, carbonate or bicarbonate or ammonium formate or ammonia with,possibly aqueous, formic acid, subsequently cooling the reactionmixture, filtering the resultant slurry and drying the resultant filtercake and recirculating the filtrate.

A disadvantage of the abovementioned processes is that, per mole offormate formed by the reaction with the basic compounds, in each caseone mole of formic acid is consumed. This is because, as is known, it isprecisely the preparation of concentrated, that is to say substantiallyanhydrous, formic acid, which is a process which requires extensiveequipment, and is costly and energy-consuming. Thus the abovementionedprocesses, based on the entire value-added chain, require extensiveequipment and are costly and energy-consuming.

German application No. 102 10 730.0 teaches preparing acid formates byreacting methyl formate with water and a basic compound having a PK_(a)of the conjugate acid of the appropriate dissociation state of ≧3, andsubsequently removing the methanol formed and optionally setting thedesired acid content by adding formic acid.

German application No. 101 54 757.9 teaches preparing metalformate/formic acid mixtures by carbonylating the corresponding metalhydroxide to give the metal formate in the presence of water and acatalyst, removing the water and the catalyst by distillation and addingformic acid to the metal formate to produce the desired metalformate/formic acid mixture.

It is an object of the present invention, therefore, to provide aprocess which no longer has the abovementioned disadvantages, whichmakes it possible to prepare acid formates on an industrial scale inhigh yield and high space-time yield, with simultaneously highflexibility with respect to composition and with the use of readilyaccessible raw materials and which permits a simple process procedurewith low capital costs and low energy consumption.

We have found that this object is achieved by a process for preparingacid formates, which comprises

-   -   (a) partially hydrolyzing methyl formates with water;    -   (b) separating off by distillation methyl formate and methanol        from the reaction mixture obtained in process stage (a), forming        a stream comprising formic acid and water; and    -   (c) combining the stream comprising formic acid and water from        the process stage (b) with the corresponding formate, forming a        mixture comprising the acid formate and water.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a process flow chart of the instant invention.

Acid formates are compounds and mixtures which contain formate anions(HCOO⁻), cations (M^(x+)) and formic acid (HCOOH). They can occurtogether in the form of a solid or a liquid and if appropriate containother components, for example other salts, additives or solvents such aswater. Generally, the acid formates can be represented by the formulaHCOO⁻ M ^(x+) _(1/x) *yHCOOH  (I),where M is a monovalent or polyvalent inorganic or organic cation, x isa positive integer and indicates the charge of the cation and y givesthe molar fraction of formic acid based on the formate anion. The molarfraction of formic acid based on the formate anion y is generally from0.01 to 100, preferably from 0.05 to 20, particularly preferably from0.5 to 5, and in particular from 0.9 to 3.1.

The nature of the inorganic or organic cation M^(x+) is in principle notcritical, provided that this is stable under conditions under which theacid formate is to be handled. This also includes, for example,stability toward the reducing formate anion. Possible inorganic cationsare the monovalent and/or polyvalent metal cations of metals of groups 1to 14 of the Periodic Table of the Elements, for example lithium (Li⁺),sodium (Na⁺), potassium (K⁺), cesium (Cs⁺), magnesium (Mg²⁺), calcium(Ca²⁺), strontium (Sr²⁺) and barium (Ba²⁺), preferably sodium (Na⁺),potassium (K⁺), cesium (Cs⁺) and calcium (Ca²⁺). Possible organiccations are unsubstituted ammonium (NH₄ ⁺) and ammonium substituted byone or more carbon-containing radicals which may also be linked to oneanother, for example methylammonium, dimethylammonium,trimethylammonium, ethylammonium, diethylammonium, triethylammonium,pyrollidinium, N-methylpyrroldinium, piperidinium, N-methylpiperidiniumor pyridinium.

A carbon-containing organic radical is an unsubstituted or substituted,aliphatic, aromatic or araliphatic radical having from 1 to 30 carbons.This radical can contain one or more heteroatoms, such as oxygen,nitrogen, sulfur or phosphorus, for example —O—, —S—, —NR—, —CO—, —N═,—PR— and/or —PR₂ and/or be substituted by one or more functional groupswhich, for example, contain oxygen, nitrogen, sulfur and/or halogen, forexample by fluorine, chlorine, bromine, iodine and/or a cyano group (theradical R here is also a carbon-containing organic radical). Thecarbon-containing organic radical can be a monovalent or polyvalentradical, for example divalent or trivalent radical.

The individual process stages are described in more detail below:

Process Stage (a)

In process stage (a), methyl formate is partially hydrolyzed with waterto formic acid and methanol. Partially means that only a portion of themethyl formate fed is hydrolyzed.

In the inventive process, in process stage (a) processes which are knownper se for hydrolyzing methyl formate can be used. A general overview ofknown and industrially relevant processes for hydrolysis is given, forexample, in Ullmann's Encyclopedia of Industrial Chemistry, 6^(th)edition, 2000 electronic release, Chapter “FORMIC ACID, Production”.Other suitable hydrolysis processes are also described, for example, inEP-A 0 005 998 and EP-A 0 017 866.

The hydrolysis is generally carried out at a temperature of from 80 to150° C. and a pressure of from 0.5 to 2.0 MPa absolute. Reactionapparatuses which can be used are in principle all reaction apparatuseswhich are suitable for reactions in the liquid phase. Examples arestirred tanks and jet loop reactors. Preference is given to the use of acascade reactor.

Generally, it is advantageous to carry out the hydrolysis in thepresence of an acid catalyst, since this significantly increases thehydrolysis rate. Acid catalysts which can be used are the formic acidwhich is formed or additional catalysts. The additional catalysts can beof homogeneous or heterogeneous nature. Examples of heterogeneouscatalysts are acid ion-exchangers, for example polysulfonic acids orpoly(perfluoroalkylene)sulfonic acids (for example Nafion® from Du Pont)and examples of homogeneous catalysts are strong inorganic or organicacids, such as sulfuric acid, hydrochloric acid or alkyl- andtolylsulfonic acids. If homogeneous catalysts are used, these mustgenerally be removed in a subsequent stage. Depending on the desiredpurity of the acid formates to be prepared, however, it is also possibleto allow these to remain in the system. In this case, the acid catalystsare usually recovered in the form of their salts in the acid formate.Particularly preferably, the partial hydrolysis is carried out in thepresence of formic acid as acid catalyst, which avoids adding anadditional catalyst and its subsequent removal or possible contaminationof the acid formates. Generally, for this purpose, at the reactor inleta formic acid concentration of from about 0.1 to 2% by weight, based onthe liquid mixture present which contains water and methyl formate, isestablished, by targeted addition of formic acid or a stream comprisingformic acid.

The molar ratio of methyl formate to water to be used in the hydrolysisin the inventive process is generally from 0.1 to 10. Since this is anequilibrium reaction, an excess of water is preferably used, as alsofollows, for example, from the teaching of EP-A 0 017 866. Preferably inprocess stage (a), the methyl formate and the water are fed in a molarratio of from 0.1 to 1, and particularly preferably from 0.15 to 0.3.

The reaction mixture obtained from the partial hydrolysis thus comprisesunreacted methyl formate, formic acid, methanol and, owing to thepreferred use of an excess of water, water. Preferably, the aqueousreaction mixture comprises from 5 to 15 mol %, particularly preferablyfrom 8 to 12 mol %, formic acid, from 3 to 10 mol %, particularlypreferably from 6 to 12 mol %, methyl formate and from 6 to 15 mol %,particularly preferably from 8 to 12 mol %, methanol.

Process Stage (b)

In process stage (b), methyl formate and methanol are removed bydistillation from the reaction mixture obtained in process stage (a),forming a stream comprising formic acid and water. Methyl formate andmethanol can here in principle be removed together in the form of astream or separately in the form of a stream comprising methyl formateand a stream comprising methanol. Generally, methyl formate and methanolare taken off separately or together in the upper part of the column.The stream comprising formic acid and water is generally taken off fromthe bottom. Preference is given in process stage (b) to the jointremoval of a stream comprising methyl formate and methanol.

The design and operation of the distillation column is primarilydependent on the composition of the stream which is fed and on thedesired purities of the two product streams and can be determined in aknown way by those skilled in the art.

The methyl formate separated off in process stage (b) is preferablyrecirculated to process stage (a). If, in process stage (b), as ispreferred, methyl formate and methanol are separated off together in theform of a joint stream, preferably the methyl formate, before it isrecirculated, is substantially freed from methanol by distillation. Thisis generally performed in a column downstream of the column of processstage (b). Since methyl formate is generally prepared by carbonylatingmethanol, it is particularly advantageous to recirculate the streamcomprising the remaining methanol as starting material for thepreparation of methyl formate, in which case the methanol to berecirculated in this variant can certainly still contain residualamounts of methyl formate. Thus it is only necessary in the overallbalance to replace the small methanol losses by fresh methanol.

Process Stage (c)

In process stage (c), the stream from process stage (b) which comprisesformic acid and water is combined with the corresponding formate,forming a mixture comprising the acid formate and water.

The formates to be used can generally be represented by the formula (II)HCOO⁻M^(x+) _(1/x)  (II),where M and x have the meaning specified under (I). Preferably, sodiumformate, potassium formate and/or calcium formate is used in theinventive process, and particularly preferably sodium formate and/orpotassium formate.

The manner in which the formates to be used are added is generally notcritical in the inventive process. They can be added in solid or liquidform, as pure substance, as mixture of substances or as solution.Examples are addition in the form of aqueous solutions (for exampleaqueous solutions of the alkali metal formates) and addition in the formof solid compounds (for example powders of the alkali metal formates).Preference is given to addition in the form of aqueous solutions.

The order in which the stream from the process stage (b) which comprisesformic acid and water and the corresponding formate are added isgenerally not critical in the inventive process. In particular, it ispossible and may be advantageous, before they are combined, toconcentrate formic acid in the stream from process stage (b) whichcomprises formic acid and water. In this context, mention may be made,in particular, of removing a portion of the water present byevaporation, preferably by distillation.

Temperature and pressure are generally not critical for the combiningoperation in process stage (c). Generally they are combined at atemperature of from 0 to 150° C. and a pressure of from 0.01 to 0.3 MPaabsolute.

Apparatuses which can be used are in principle all appratuses which aresuitable for reactions in the liquid phase and, if appropriate, forreactions in the liquid phase with simultaneous removal of a volatilecomponent. Examples are stirred tanks, jet loop reactors and columns. Inaddition, it is also possible, for example, to combine the two streamsby their meeting within a pipe, advantageously having a downstreammixing section. In addition, it is also possible to combine the twostreams in the apparatus in which solid acid formate is also isolated.

The mixture obtained by combining the stream comprising the formic acidand the water from the process stage (b) and the corresponding formatecomprises the acid formate in the form of an aqueous solution, with orwithout previously precipitated acid formate as solid. Depending onrequirements, it can in this form be packaged, stored, transportedand/or used for appropriate formulations or uses. In addition, the acidformate can be further concentrated or isolated as solid by downstreamprocess steps.

Preference is given to a variant in which, in the process stage (c)

-   -   (i) the stream comprising the formic acid and the water from        process stage (b), together with the mother liquor recirculated        from step (iv) is concentrated in a column or an evaporator with        removal of water by distillation;    -   (ii) the stream which was produced from step (i) by        concentration and which comprises formic acid, water and formate        is combined with the corresponding formate forming a mixture        comprising the acid formate and water;    -   (iii) solid acid formate from the mixture comprising acid        formate and water which is obtained from step (ii) is        precipitated by crystallization and this is isolated; and    -   (iv) The resultant mother liquor is recirculated to step (i).

The column or the evaporator in step (i) is generally to be operated insuch a manner that a portion of the water fed can be taken off, forexample overhead. The remaining stream comprising formic acid, water andformate generally has a water content of from 10 to 40% by weight and iswithdrawn as bottoms product. Said procedure has the advantage of acertain concentration of the stream comprising the formic acid and theformate. The water withdrawn from the column or the evaporator isadvantageously recirculated to the hydrolysis stage in process step (a)and the excess is taken off from the process. The column or evaporatoris designed in a manner known and customary to those skilled in the art.

The stream which is produced by concentration and comprises formic acid,water and formate can be combined with the corresponding formate forminga mixture comprising the acid formate and water in step (ii), forexample, between the column and the crystallization apparatus, forexample by combining two lines, or they can be combined in a separatemixing apparatus, or in the crystallization apparatus itself. Thecorresponding formate in this case is preferably used as aqueoussolution.

The crystallization procedure is generally known to those skilled in theart, with the precise design and procedure being able to be performed inthe customary manner. Generally, the crystallization is carried out at atemperature in the range from −20° C. to +80° C., and preferably from 0°C. to 60° C. Generally, the amount of product crystallized out increaseswith decreasing temperature. The crystallization can in principle becarried out in all apparatuses known for this purpose. Said embodimentis particularly advantageously usable for removing acid formates whichcan crystallize in the desired composition. Relevant examples arepotassium diformate (HCOOK.HCOOH), sodium diformate (HCOONa.HCOOH),sodium tetraformate (HCOONa.3HCOOH) or mixtures thereof. The formates oracid formates which have crystallized out are generally removed bycustomary and known methods, for example by filtration orcentrifugation.

The mother liquor which is produced in the crystallization of the solidacid formate is recirculated in step (iv) to step (i). Since this motherliquor still comprises a considerable proportion of product of value,isolation thereof is thus also ensured. However, alternatively, it isalso possible to use the product of value present in the motehr liquorin another manner, for example by direct use as solution.

Likewise, preference is given to a variant in which, in process stage(c)

-   -   (i) the stream from the process stage (b) comprising the formic        acid and the water is combined with the corresponding formate to        form a mixture comprising the acid formate and water in a column        or an evaporator with removal of water by distillation; and    -   (ii) solid acid formate is separated off by spray granulation,        spray-drying or melt crystallization from the mixture obtained        from step (i) comprising acid formate and water, and this solid        acid formate is isolated.

The two streams can be combined in step (i) upstream of the column orthe evaporator, for example by joining two lines, or they can becombined in a seprate mixing apparatus or in the column or in theevaporator, for example via two separate feeds. The correspondingformate is preferably used as aqueous solution here.

The column or the evaporator in step (i) is generally to be operated insuch a manner that a portion of the water fed can be taken off, forexample overhead. The remaining acid-formate-containing mixture, whichgenerally has a water content of from 0.5 to 30% by weight, is withdrawnas bottoms product. In particular in the isolation of the acid formateby means of melt crystallization, a lower water content of generally ≦1%by weight is set in the bottoms product. Said procedure has theadvantage of a certain concentration of the stream comprising the acidformate. The water withdrawn from the column or the evaporator isadvantageously recirculated to the hydrolysis stage in process step (a)and the excess is taken off from the process. The column or theevaporator is designed in the manner known and customary to thoseskilled in the art.

The spray granulation, spray-drying and melt crystallization proceduresare generally known to those skilled in the art, in which case theprecise design and procedure can be carried out in the customary manner.The abovementioned methods can also particularly advantageously be usedfor removing acid formates which can be crystallized in the desiredcomposition. Relevant examples are potassium diformate (HCOOK.HCOOH),sodium diformate (HCOONa.HCOOH), sodium tetraformate (HCOONa.3HCOOH) ormixtures thereof.

Since in spray granulation, spray-drying and melt crystallization,advantageously an aqueous acid formate having a low water content can beused, generally, also, only a small proportion of condensate or freeformic acid is obtained.

Depending on the amount produced and the residual concentration of acidformate present, it may also be advantageous not to recirculate thestream, but to eject it from the system.

The corresponding formates which are to be fed to the process stage (c)in the inventive process can be prepared in the most varied manners. Ageneral overview of known and technically relevant processes forpreparing formates is given, for example, in Ullmann's Encyclopedia ofIndustrial Chemistry, 6^(th) edition, 2000 electronic release, Chapter“FORMIC ACID, Derivatives, Salts”. Further suitable preparationprocesses are also described, for example, in U.S. Pat. No. 3,262,973.

The inventive process can be carried out in principle batchwise,semicontinuously or continuously. Preferably, the inventive process iscarried out continuously.

Preferably, in the inventive process, the acid formate prepared is acidmetal formates, particularly preferably acid potassium formate, acidsodium formate, acid calcium formate or mixtures thereof and veryparticularly preferably potassium diformate (HCOOK.HCOOH), sodiumdiformate (HCOONa.HCOOH), sodium tetraformate (HCOONa.3HCOOH) ormixtures thereof.

The acid formates are generally prepared in the form of their solutions,or crystalline as solids. If appropriate, they can further be admixedwith other components, for example other formate salts. In the case ofthe crystalline acid formates, it is generally advantageous for storage,transport and use to compact these together with a desiccant, forexample silicates or starch, to form a particulate compactate or diverseshaped bodies, for example tablets or beads.

Preferably, in the inventive process, the acid formate prepared is anacid metal formate and the metal formate to be supplied in the processstage (c) is prepared by carbonylating the corresponding metalhydroxide.

The acid metal formate generally comprises, as possible inorganiccations, the monovalent and/or polyvalent metal cations of the metalsfrom groups 1 to 14 of the Periodic Table of the Elements, for examplelithium (Li⁺), sodium (Na⁺), potassium (K⁺), cesium (Cs⁺), magnesium(Mg²⁺), calcium (Ca²⁺), strontium (Sr²⁺) and barium (Ba²⁺), andpreferably sodium (Na⁺), potassium (K⁺), cesium (Cs⁺) and calcium(Ca²⁺).

Said carbonylation proves particularly advantageous, in particular sinceit makes it possible to use readily and simply accessible startingmaterials and is technically simple to carry out. Thus, for example, inaccordance with A. F. Hollemann, N. Wiberg, Lehrbuch der anorganischenChemie [Textbook of Inorganic Chemistry], Walter de Gruyter VerlagBerlin New York, 1985, 91st-100th edition, page 722 sodium formate maybe prepared by introducing carbon monoxide into sodium hydroxidesolution at from 150 to 170° C. and a pressure of from 3 to 4 bar, andin accordance with page 947 of said textbook, potassium formate may beprepared by the action of carbon monoxide on an aqueous solution ofpotassium sulfate and lime at 230° C. and 30 bar. According to Ullmann'sEncyclopedia of Industrial Chemistry, 6^(th) edition, 2000 electronicrelease, Chapter “FORMIC ACID, Prodution, Other Processes”, sodiumformate can be produced, for example, by the action of carbon monoxideon aqueous sodium hydroxide solution at 180° C. and from 1.5 to 1.8 MPa,using a reaction tower. The aqueous sodium hydroxide solution tricklesin this case from the top to the bottom, whereas the carbon monoxideflows in countercurrent from bottom to top.

Generally, the preferred carbonylation to give the corresponding metalformates is performed in the inventive process in the presence of acatalyst at a temperature of from 20 to 250° C., preferably from 30 to160° C., and particularly preferably from 90 to 120° C., and at apressure of from 0.1 to 12 MPa absolute, and preferably from 0.3 to 6MPa absolute.

The catalyst used is at least one catalyst selected from the groupconsisting of the alcohols and formic esters. In principle, allcatalysts are suitable in which the metal hydroxides dissolve readily.Suitable catalysts are, for example, saturated unbranched C₁-C₄alkanols, unsaturated unbranched C₃-C₄ alkanols, saturated branchedC₃-C₄ alkanols and unsaturated branched C₄ alkanols, and formic estersthereof. If an alcohol in a mixture with a formic ester is used ascatalyst, generally the formic ester of this alcohol is used. Preferenceis given to use of saturated unbranched C₁-C₄ alkanols and saturatedbranched C₃-C₄ alkanols, particularly preferably methanol. The catalystis generally used at a concentration of from 1 to 40% by weight,preferably from 5 to 25% by weight, particularly preferably from 10 to20% by weight, based on the total reaction solution.

Compared with otherwise customary processes for formate preparation,operations can be carried out at higher concentration ranges of themetal hydroxide and with a trend toward higher pressures and lowertemperatures. Since the reaction is limited by mass transfer, higherspace-time yields can be achieved by good mixing, for example usingmixing nozzles.

The reaction can be carried out either continuously or batchwise.Preference is given to a continuous reaction. Generally, the reaction iscarried out in such a manner that the metal hydroxide is convertedvirtually quantitatively to the metal formate. The reaction isadvantageously carried out until the content of metal hydroxide in thereaction solution is less than 0.1% by weight, preferably less than0.04% by weight, particularly preferably less than 0.01% by weight.

The reaction can in principle be carried out in any type of reactionapparatus. Preferably, it is carried out in a stirred tank having agas-introduction device, in a bubble column or in a loop reactor.Particularly preferably, the reaction is carried out in a loop reactoror a bubble column, very particularly in a loop reactor, since in thiscase, on account of the high interface area between the metal hydroxideand catalyst-containing water-containing solution and the carbonmonoxide introduced, a high absorption rate results, and thus also ahigh reaction rate. When a bubble column is used, the carbon monoxidecan be fed, for example, in the upper region (cocurrent procedure) orelse in the lower region (countercurrent procedure).

The metal hydroxides are generally used as aqueous solution. Theconcentration of these metal hydroxide solutions is generally from 25 to50% by weight, preferably from 45 to 50% by weight, particularlypreferably 48.5 to 50% by weight. The aqueous soltuion can also comprisea plurality of metal hydroxides. Generally, no special requirements aremade of the purity of the metal hydroxide solutions used. Therefore,generally, technical grade metal hydroxide solutions can be used. Thepreferred inventive process may also be carried out using pure metalhydroxide solutions. Preference is given to the hydroxides of sodium,potassium and/or calcium.

Carbon monoxide can be used not only as individual component, but alsoin a mixture with other gases, for example nitrogen or noble gases. Ifcarbon monoxide is used in a mixture with other gases, the content ofcarbon monoxide in the gas mixture should be at least 5% by volume,preferably at least 10% by volume, particularly preferably at least 25%by volume, and very particularly preferably at least 50% by volume. Thecarbon monoxide partial pressure during the reaction should generally befrom 0.1 to 12 MPa, and preferably from 2 to 6 MPa. Generally, nospecial requirements are made of the purity of the carbon monoxide orcorresponding carbon-monoxide-containing gas mixture used. The reactionmay therefore be carried out not only with pure carbon monoxide, butalso with technical grade carbon monoxide or carbon-monoxide-containinggas mixtures.

Said reaction procedure ensures that crystallization or precipitation ofmetal formate does not occur, since the reaction mixture is present assolution for the period of the reaction and precipitation of solids andblockage of piping are avoided.

The metal formates obtained by the carbonylation described are generallypresent in the reaction solution at a concentration of from 10 to 90% byweight, preferably from 30 to 80% by weight, and particularly preferablyfrom 40 to 70% by weight.

Particularly preferably (i), in the inventive process, the carbonylationis carried out in the presence of methanol as catalyst, (ii) theresultant reaction mixture comprising metal formate, water and methanol,together with the stream from the process stage (b) comprising methanolwith or without methyl formate are separated by distillation into amethanol-containing stream, if appropriate into a stream comprisingmethyl formate, and a stream comprising the metal formate and water, and(iii) the resultant stream comprising the metal formate and water is fedto process stage (c).

Preferably, step (ii) is carried out in a column by feeding as feed thestream from the process stage (b) comprising methanol with or withoutmethyl formate and feeding, beneath the abovementioned feed, thereaction mixture comprising the metal formate, water and methanolresulting from the carbonylation. The low-boiling components methylformate and methanol ascend, whereas the metal formate and water descendand are withdrwan as bottoms product. The lowest-boiling component iswithdrawn as overhead product. If, from the process stage (b) a streamcomprising methanol and also methyl formate is fed, as is particularlypreferred, a stream comprising methyl formate is withdrawn as overheadproduct from the column. This stream is preferably recirculated to theprocess stage (a) for hydrolysis. The methanol-containing stream isproduced in this case as a side stream in the upper region of thecolumn. Since methyl formate is generally prepared by carbonylatingmethanol, it is particularly advantageous to recirculate themethanol-containing stream as starting material for the preparation ofmethyl formate, in which case the methanol to be recirculated in thisvarient can certainly also comprise residual amounts of methyl formate.Thus, in the overall balance, it is only necessary to replace the smallmethanol losses by fresh methanol.

It must be emphasized that in the described separation and recirculationof the methanol-containing stream for the preparation of methyl formate,further steps can be provided as intermediates. Thus, if appropriate, itis advantageous, in step (ii) to produce a methanol stream stillcomprising methyl formate and to separate this, in a downstream column,from the residual methyl formate, with this generally also beingrecirculated for hydrolysis to the process stage (a). Themethanol-containing bottoms product of this downstream column is thengenerally fed to the methyl formate preparation, in which case theamount of methanol required to catalyze the carbonylation can be fed tothe carbonylation reactor.

In a particularly preferred embodiment, the simplified process flowchart which is shown in FIG. 1, via line (1), methyl formate and watercomprising formic acid which is recirculated from the process are addedto the cascade hydrolysis reactor (A). Generally, the two startingmaterials are brought to the desired inlet temperature in a heatexchanger premixed (as shown in the flow chart) or separately. Thereaction mixture originating from the hydrolysis stage (process stage(a)), which reaction mixture comprises unreacted methyl formate, water,formic acid and methanol, is fed via line (2) to the column (B) in whichthe reaction mixture is separated by distillation into an overheadstream comprising methyl formate and methanol, and a bottoms streamcomprising aqueous formic acid (process stage (b)). The overhead streamcomprising methyl formate and methanol is fed via line (3) to column(C). In addition, the reaction mixture comprising metal formate, waterand methanol from the carbonylation is fed to the column (C) beneath theinlet point of the stream comprising methyl formate and methanol vialine (12). Methyl formate is obtained overhead from column (C) via line(4) and is recirculated to the process stage (a) for the hydrolysis. Amethyl-formate-containing methanol stream is obtained via a sidestreamtakeoff in the upper region of the column and is fed via line (6) to thecolumn (D). In this column the stream is separated into a methyl formateoverhead stream which is recirculated via line (7) to the process stage(a) for the hydrolysis also, and a methanol bottoms stream, which isrecirculated via line (5) for the preparation of methyl formate, withthe amount of methanol required to catalyze the carbonylation being fedto the carbonylation reactor. The carbonylation for preparing thecorresponding formate is performed in reactor (H). To this is fedaqueous metal hydroxide, particularly preferably potassium hydroxidesolution, via line (8), and carbon monoxide is fed via line (9). Line(10) serves primarily for retaining pressure and if appropriate forejecting the purge stream. At the bottom end of column (C), a portion ofthe water is withdrawn and recirculated via line (11) to the hydrolysisstage. The bottoms product obtained is an aqueous metal formatesolution. The stream comprising aqueous formic acid from process stage(b) is fed via line (14) to the column (E). If appropriate, a portion ofthe aqueous metal formate solution from column (C) is also fed via lines(13) and (13 b). The column (E) is advantageously operated in such amanner that the bottoms product obtained is a concentrated mixturecomprising formic acid, metal formate and water having a water contentof generally from 10 to 40% by weight. A portion of the water iswithdrawn from the column (E) in the form of a formic-acid-containingwater stream as overhead product and recirculated via line (19) to thehydrolysis stage. A portion of the water stream comprising small amountsof formic acid can here optionally be withdrawn from the system via line(18). The bottoms product of column (E) is fed via line (15) to anapparatus (G) suitable for crystallization, for example a cooling disccrystallizer. The aqueous metal formate solution from column (C) is fedvia line (13 a). The feed in this case can be performed, for example, inthe lower region of column (E), by combining two lines (as shown inFIG. 1) or directly into the crystallization apparatus. Thecrystallization is primarily performed by temperature decrease. Theresultant crystals are fed together with the supernatant solution forseparation to the apparatus (F). Preferably the separation is performedby centrifugation. The separated crystals are withdrawn via line (16)and can be dried and/or compounded, for example in optional followingstages. The resultant mother liquor is recirculated via line (17) to thecolumn (E).

In another particularly preferred embodiment, whose simplified processflow chart is shown in FIG. 2, the process stages (a) and (b) and thepreparation of the metal formate, preferably of the potassium formate,and the operation of the columns (C) and (D) are carried out asdescribed in the above particularly preferred embodiment. The streamcomprising the aqueous formic acid from the process stage (b) is fed vialine (14) and the stream comprising the aqueous metal formate solutionfrom column (C) is fed via line (13) to the column (E). The column (E)is advantageously operated in such a manner that the bottoms productobtained is a concentrated mixture comprising formic acid, metal formateand water having a water content generally from 0.5 to 30% by weight. Aportion of the water fed is withdrawn from the column (E) as overheadproduct in the form of a formic-acid-containing water stream and isrecirculated via line (19) to the hydrolysis stage. A portion of thewater stream comprising small amounts of formic acid can here optionallybe withdrawn from the system via line (18). The bottoms product of thecolumn (E) is fed via line (15) to an apparatus (G) suitable for spraygranulation, spray drying or melt crystallization. The resultant solidacid formate is withdrawn via line (16) and can be dried and/orcompounded, for example in optional following stages. The resultantcondensate can optionally be recirculated via line (17) to the column(E) or ejected from the system.

The inventive process makes it possible to prepare acid formates on anindustrial scale in high yield and high space-time yield, withsimultaneously high flexibility with respect to composition and usingreadily accessible raw materials with simple process design and lowcapital costs. In addition, the process has the critical advantage thatthe required formic acid can be produced directly from the methylformate without the costly diversion, which is expensive in terms ofresources, via the concentrated formic acid, whereas the requiredformate can be obtained for example in a simple manner by carbonylationusing easily accessible starting materials. The inventive process istherefore simple to carry out in processing terms and compared with theprocesses involving direct use of concentrated formic acid according tothe prior art, has markedly lower capital costs and a markedly lowerenergy consumption. In addition, in part the use of high-alloy steelscan be avoided, since the acid formates are much less corrosive thanconcentrated formic acid.

In addition, the invention relates to an apparatus for preparing theacid formates according to the inventive process, comprising

-   -   (a) a reactor (A) suitable for hydrolyzing methyl formate;    -   (b) a column (B) suitable for separating by distillation a        stream comprising methyl formate, formic acid, methanol and        water into methyl formate, methanol and a stream comprising        formic acid and water, which column is connected on the feed        side to the reactor (A);    -   (c) a column (E) suitable for removing water from a stream        comprising formic acid and water, which column is connected on        the feed side to the column bottom of column (B).

A suitable reactor (A) is, for example, a stirred tank or a jet loopreactor. Preference is given to a cascade reactor. The reactor (A) isdesigned according to the manner customary and known to those skilled inthe art.

The columns (B) and (E) are designed in the manner which is customaryand known to those skilled in the art.

A preferred apparatus is an apparatus which, in addition to theabovementioned features (a) to (c), comprises

-   -   (d) an apparatus (G) suitable for crystallizing acid formate,        which apparatus is connected on the feed side to the column        bottom of column (E) and to a possible supply of aqueous        formate;    -   (f) an apparatus (F) suitable for separating off crystals of the        acid formate, which apparatus is connected on the feed side to        apparatus (G); and    -   (g) a connection line (17) between apparatus (F) and column (E),        which connection line is suitable for recirculating mother        liquor.

The apparatuses (G) and (F) are designed in the manner which iscustomary and known to those skilled in the art.

Furthermore, the preferred apparatus is an apparatus which, in additionto the abovementioned features (a) to (c), comprises

-   -   (e) a possible supply to the column (E), which possible supply        is suitable for feeding aqueous formate; and    -   (f) an apparatus (G) suitable for spray granulation, spray        drying or melt crystallization, which apparatus is connected on        the feed side to the column bottom of column (E).

The apparatus (G) is designed in the manner which is customary and knownto those skilled in the art.

In addition, the invention relates to the use of the inventivelyprepared acid formates for preserving and/or acidifying plant and animalmaterials. Examples are the use of acid formates for preserving andacidifying grass, agricultural plants, fish and fish products and meatproducts, as are described, for example, in WO 97/05783, WO 99/12435, WO00/08929 and WO 01/19207.

Furthermore, the invention relates to the use of the inventivelyprepared acid formates for treating biowastes. The use of acid formatesfor treating biowastes is described, for example, in WO 98/20911.

In addition, the invention relates to the use of the inventivelyprepared acid formates as additive in animal nutrition and/or as growthpromoters for animals, for example for breeding sows, growing/finishingpigs, poultry, calves, cows and fish. Said use is described, forexample, in WO 96/35337. Preference is given to the use of theinventively prepared acid potassium formates, in particular potassiumdiformate, as additive in animal nutrition and/or as growth promotersfor animals, in particular for breeding sows and growing/finishing pigs.

Very particularly preferred mixtures for the preferred use of the acidpotassium formates prepared by the inventive process as additive inanimal nutrition and/or as growth promoters for animals are thefollowing two compositions:

Mixture 1 Mixture 2 (% by (% by weight) weight) Potassium diformate 20to 60 60 to 99 Sodium diformate/tetraformate 20 to 50 — Calcium formate 0 to 25  0 to 28 Dessicant (silicate or starch) 0 to 4 0 to 4 Water 0to 5 0 to 5

Very particular preference is given to the use of the inventivelyprepared potassium diformate as additive in animal nutrition and/or asgrowth promoter for animals in the form of a product of composition98.0±1% by weight potassium diformate, 1.5±1% by weight silicate and0.5±0.3% by weight water.

1. A process for preparing acid formates comprising: (a) partiallyhydrolyzing methyl formates with water; (b) separating off bydistillation methyl formate and methanol from the reaction mixtureobtained in process stage (a), forming a stream comprising formic acidand water; and (c) combining the stream comprising formic acid and waterfrom the process stage (b) with the corresponding formate forming amixture comprising the acid formate and water.
 2. A process according toclaim 1, wherein, in the process stage (a), the methyl formate and thewater are fed in a molar ratio of 0.1 to
 1. 3. A process according toclaim 1, wherein the methyl formate separated off in process stage (b)is recirculated to process stage (a).
 4. A process according to claim 1,wherein, in the process stage (d): (i) the stream comprising the formicacid and the water from the process stage (b), together with the motherliquor recirculated from step (iv), is concentrated in a column or anevaporator with removal of water by distillation; (ii) the stream whichwas produced from step (i) by concentration and comprises formic acid,water and formate is combined with the corresponding formate, forming amixture comprising the acid formate and water; (iii) solid acid formatefrom the mixture comprising acid formate and water obtained from step(ii) is precipitated by crystallization and this is isolated; and (iv)the resultant mother liquor is recirculated to step (i).
 5. A processaccording to claim 1, wherein, in process stage (c): (i) the stream fromthe process stage (b) comprising the formic acid and the water and thecorresponding formate are combined to form a mixture comprising the acidformate and water in a column or an evaporator with removal of water bydistillation; and (ii) solid acid formate is separated off by spraygranulation, spray drying or melt crystallization from the mixtureobtained from step (i) comprising acid formate and water, and this solidacid formate is isolated.
 6. A process according to claim 1, wherein theacid formate prepared is an acid metal formate and the metal formate tobe supplied in process stage (c) is produced by carbonylating thecorresponding metal hydroxide.
 7. A process according to claim 6,wherein: (i) the carbonylation is carried out in the presence ofmethanol as catalyst; (ii) the resultant reaction mixture comprisingmetal formate, water and methanol together with the stream comprisingmethanol with or without methyl formate from the process stage (b) isseparated by distillation into a stream comprising methanol, with orwithout a stream comprising methyl formate and a stream comprising themetal formate and water; and (iii) the resultant stream comprising themetal formate and water is fed to the process stage (c).
 8. A processaccording to claim 1, wherein the acid formate prepared is selected fromthe group consisting of acid potassium formate, acid sodium formate,acid calcium formate and mixtures thereof.
 9. A process according toclaim 1, wherein the acid formate prepared is selected from the groupconsisting of potassium diformate, sodium diformate, sodium tetraformateand mixtures thereof.
 10. A process according to claim 2, wherein themethyl formate separated off in process stage (b) is recirculated toprocess stage (a).
 11. A process according to claim 2, wherein, in theprocess stage (d): (i) the stream comprising the formic acid and thewater from the process stage (b), together with the mother liquorrecirculated from step (iv), is concentrated in a column or anevaporator with removal of water by distillation; (ii) the stream whichwas produced from step (i) by concentration and comprises formic acid,water and formate is combined with the corresponding formate, forming amixture comprising the acid formate and water; (iii) solid acid formatefrom the mixture comprising acid formate and water obtained from step(ii) is precipitated by crystallization and this is isolated; and (iv)the resultant mother liquor is recirculated to step (i).
 12. A processaccording to claim 3, wherein, in the process stage (d): (i) the streamcomprising the formic acid and the water from the process stage (b),together with the mother liquor recirculated from step (iv), isconcentrated in a column or an evaporator with removal of water bydistillation; (ii) the stream which was produced from step (i) byconcentration and comprises formic acid, water and formate is combinedwith the corresponding formate, forming a mixture comprising the acidformate and water; (iii) solid acid formate from the mixture comprisingacid formate and water obtained from step (ii) is precipitated bycrystallization and this is isolated; and (iv) the resultant motherliquor is recirculated to step (i).
 13. A process according to claim 2,wherein, in process stage (c): (i) the stream from the process stage (b)comprising the formic acid and the water and the corresponding formateare combined to form a mixture comprising the acid formate and water ina column or an evaporator with removal of water by distillation; and(ii) solid acid formate is separated off by spray granulation, spraydrying or melt crystallization from the mixture obtained from step (i)comprising acid formate and water, and this solid acid formate isisolated.
 14. A process according to claim 3, wherein, in process stage(c): (i) the stream from the process stage (b) comprising the formicacid and the water and the corresponding formate are combined to form amixture comprising the acid formate and water in a column or anevaporator with removal of water by distillation; and (ii) solid acidformate is separated off by spray granulation, spray drying or meltcrystallization from the mixture obtained from step (i) comprising acidformate and water, and this solid acid formate is isolated.
 15. Aprocess according to claim 2, wherein the acid formate prepared is anacid metal formate and the metal formate to be supplied in process stage(c) is produced by carbonylating the corresponding metal hydroxide. 16.A process according to claim 3, wherein the acid formate prepared is anacid metal formate and the metal formate to be supplied in process stage(c) is produced by carbonylating the corresponding metal hydroxide. 17.A process according to claim 4, wherein the acid formate prepared is anacid metal formate and the metal formate to be supplied in process stage(c) is produced by carbonylating the corresponding metal hydroxide. 18.A process according to claim 5, wherein the acid formate prepared is anacid metal formate and the metal formate to be supplied in process stage(c) is produced by carbonylating the corresponding metal hydroxide.